PERF: use labels to find duplicates in multi-index

[behzadnouri]

-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
multiindex_duplicated                        | 139.2883 | 469.6666 |   0.2966 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [2b99b80] : use labels to identify duplicates in multi-index
Base   [0fe43a6] : Merge pull request #9120 from minrk/nbviewer-link

fix an nbviewer link in tutorials

[jreback]

this on top of #9061 ?

[jreback]

sequence

[behzadnouri]

get_flat_ids is copy pasted from #9061. MultiIndex.is_unique has been re-factored.

[jreback]

ok, #9061 merged, so pls rebase this

[jreback]

pls add a release note as well

[behzadnouri]

rebased and added release note. travis build all green.

[behzadnouri]

@jreback actually i think it is better to have duplicated as a separate function as opposed to class method. so plz do not merge, and let me change the code.

[jreback]

@behzadnouri no prob. lmk

[behzadnouri]

@jreback this is done!

[jreback]

merged via 8f4a321 thanks!